The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In general, large amounts of harmful substances to humans such as carbon monoxide and nitrogen oxides are contained in exhaust gases emitted from a vehicle engine. Strict regulations are being enforced on nitrogen oxides because the nitrogen oxides are particularly harmful in that they contribute to acid rain, global warming, and respiratory problems.
The nitrogen oxides have the property that, as the combustion temperature of fuel in the engine increases, so does the amount of nitrogen oxides.
Many attempts have been made to reduce nitrogen oxide emissions, among which an exhaust gas recirculation (EGR) system is usually applied to vehicles.
The EGR system recirculates part of the exhaust gas emitted from the engine after fuel combustion to an intake system of the engine to direct it back to a combustion chamber of the engine. As a consequence, an air-fuel mixture decreases in density without a change in the air-fuel ratio of then air-fuel mixture, thus lowering the combustion temperature.
That is, the EGR system supplies part of exhaust gas to an intake manifold of the engine to direct it to the combustion chamber when there is a need to reduce nitrogen oxide emissions depending on the operating state of the engine. By doing so, exhaust gases, which are insert gases whose volume does not change, contribute to decrease the density of the mixture to a lower level and therefore decrease the flame propagation velocity during fuel combustion. This suppresses an increase in combustion temperature and slows the fuel combustion, thereby suppressing the generation of nitrogen oxides.
The EGR system according to a prior art as described above is installed between exhaust system and intake system and supplies exhaust gas exhausted from the engine along exhaust passage as needed. Further, the EGR system further includes an EGR cooler, and the EGR cooler cools exhaust gas temperature supplied to the intake system by heat exchange.
The EGR system includes an EGR valve device, and the EGR valve device includes an actuator controlled by electric control unit, an operation rod transmitting operating power of the actuator and a flap valve being rotated by the operation rod and opening and closing a port connected to the EGR cooler.
The flap valve is installed to be rotatable by means of a valve shaft to a valve housing, and the valve shaft is connected with the operation rod by a lever.
The valve shaft is integrally attached to the lever by metal inert gas welding (MIG) generally and rotatably connected to flap valve by MIG welding and riveting integrally.
By the way, exhaust gas is minutely leaked through a gap between outer diameter of the valve shaft and the lever during valve opening and closing.